... which tests reading, math, and science competency in 15-year-olds.
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A 15-y/o doesn’t even have a fully developed brain, as that doesn’t happen until around the mid-20s. What an idiotic so-called study! It should have tested people around 30.
IQ holds remarkably constant, among cohorts, as they age. The only factors likely changing the rankings in adulthood could be the relative immigration or emigration of smart people within an individual country.